The present invention relates to a permanent magnet dynamo electric machine and, in particular, to a permanent magnet dynamo electric machine having embedded permanent magnets inside the rotor thereof.
There are two types of conventional permanent magnet dynamo electric machines. In one type, the permanent magnets are secured on the circumference of the rotor thereof, and in the other type the permanent magnets are embedded inside the rotor thereof. JP-A-5-76146(1993) discloses a structure of the latter type.
In the permanent magnet dynamo electric machine having a structure of the latter type, when the rotor thereof is rotating, centrifugal forces acting on the respective permanent magnets are applied to portions of the rotor members located along the outer circumferences of the respective permanent magnets. Further, the rotor members themselves are subjected to centrifugal forces. Portions which are located at both circumferential ends of the rotor members, namely the bridge portions, support the above mentioned two sorts of centrifugal forces. Therefore, in order to withstand centrifugal forces caused by high speed rotation, the thickness of the bridge portions has to be increased.
On the other hand, when the thickness of the bridge portions is increased, magnetic fluxes generated by the permanent magnets leak via the bridge portions to the surrounding iron core, and the amount of magnetic fluxes transferred from the surface of the rotor to the stator thereof is decreased. Torque generated by the permanent magnet dynamo electric machine depends on the amount of magnetic fluxes transferred from the permanent magnets to the stator; therefore, if the magnetic flux leakage increases, the torque generated decreases and the efficiency of the permanent magnet dymano electric machine reduced accordingly.